Help regarding the alpha channel

[Psiweapon]

I need a tool that allows me to edit the alpha channel in a RGB PNG like a bitmap (I need to lay pixel-art-level partial transparencies over a pixel-art piece, to make some glass effects)

I know that I've done it a long, long time ago, but can't remember what tool did I use. Do you know of any?

[ptoing]

Photoshop can do this. Promotion can do it as well. Pretty sure that Graphics Gale might do it too.

But in PS and PM you can directly edit the alpha as a greyscale layer.

[Atnas]

You can do that in gale too, as far as the greyscale alpha goes, just make your grayscale alpha mask and then go Image -> Make Alpha Channel, and set it to luminance with the source being whatever image has the mask. Haven't used it much myself, but its there.

[Psiweapon]

You can do that in gale too, as far as the greyscale alpha goes, just make your grayscale alpha mask and then go Image -> Make Alpha Channel, and set it to luminance with the source being whatever image has the mask. Haven't used it much myself, but its there.

That sounds familiar. Is it doable in Gimp?